Volume 62, Issue 10, 15 November 1987
Index of content:

Physics of ferromagnetic amorphous alloys
View Description Hide DescriptionAfter some introductory comments on materials classification and a brief historical outline, this review seeks to identify those fundamental physical phenomena that underlie the major technical properties of ferromagneticamorphousalloys. Those phenomena are reviewed and an attempt is made to compare existing data and, where possible, to synthesize new perspectives. A fundamental approach is taken in the sense that we move from consideration of the local atomic structure to its consequences for the electronic structure and from the electronic structure finally to the physical properties it determines. We focus on the aspects of the structure‐property sequence that set amorphous metallic alloys apart from their crystalline counterparts. Such a review would be incomplete without addressing the new issues and insights raised by the discovery of the quasicrystalline state. The very existence and better understood atomic arrangements of quasicrystals help to define and illuminate their amorphous cousins.

Microfields in stroboscopic voltage measurements via electron emisson. II. Effects on electron dynamics
View Description Hide DescriptionIn this article, the transit‐time and cross‐talk effects occurring in voltage measurement via electron emission are analyzed as a function of geometry, extraction fields, electron start energies, and rise times of the actual input signals. The investigation was performed to obtain information about the maximum achievable time resolution and the disturbance of measured signals by cross‐talk effects in corresponding contactless measurement techniques like photoemissive sampling and electron‐beam probing. The field distribution above the sample was assumed as being two‐dimensional, and the influence of space‐charge effects neglected. A strong correspondence to the purely geometry‐dependent response function of the potential energy, analyzed in Part I [J. Appl. Phys. 6 2, 1553 (1987)], is established.

IBEX magnetic field extraction and propagation experiments of intense high‐energy electron beams
View Description Hide DescriptionWe have investigated the extraction and propagation of intense relativistic electron beams from an immersed foilless diode into a magnetic field‐free region. It was established experimentally that under matching conditions, the extracted beam propagates into and through the axially varying magnetic field region with minor perturbation of the equilibrium. The independent variables of the experiment were beam current, beam radius, magnetic field strength, and gas pressure in the field‐free region. The accelerator used was IBEX. [J. J. Ramirez, J. P. Corley, and M. G. Mazarakis, in P r o c e e d i n g s o f t h e F i f t h I n t e r n a t i o n a l C o n f e r e n c e o n H i g h‐P o w e r P a r t i c l e B e a m s, San Francisco (Physics International, San Leandro, CA, 1983), p. 256.] The propagation of the extracted beam was studied up to 60 cm downstream from the extraction foil. A strong, fast growing filamentation instability was observed after extraction. Time‐resolved measurements of the beam current, net current, and current distribution were performed by using segmented Faraday cups. Time‐integrated measurements of beam rotation and emittance were also recorded. The observed beam parameters and equilibrium radii before and after extraction are compared with theory and numerical calculations.

Spherical lens as a launcher of electromagnetic missiles
View Description Hide DescriptionA uniform dielectric sphere, under transient excitation by a point source, is found to be capable of launching an electromagneticmissile, i.e., there is a direction where the decay of the total energy transmitted to a receiver is slower than r ^{−} ^{2}. The conditions for the presence of an electromagneticmissile have a natural interpretation, which in turn leads to a classification of electromagneticmissiles according to the singularities of differentiable maps in two dimensions.

Effective cross section of the Nd:YAG 1.0641‐μm laser transition
View Description Hide DescriptionThe cross section of a laser transition is one of the most important parameters, because via the saturation intensity, I _{ s }=hν/(στ), the output power depends on it. For the Nd:YAG 1.0641‐μm laser transition, the published values vary between 4×10^{−} ^{1} ^{9} and 8.8×10^{−} ^{1} ^{9} cm^{2}, as obtained by spectroscopic methods. This paper deals with a new method, which determines σ from the absolute value of the output power, the small‐signal gain, and the losses. Moreover, the extraction efficiency is evaluated, taking into account the mode structure.

A perturbation calculation of the quality factor of the piezoelectric thin film on semiconductor composite resonator resulting from radiation into the wafer
View Description Hide DescriptionIn a variational analysis of the vibrations of the piezoelectricthin film on semiconductor composite resonator it was shown that the quality factor (Q) due to radiation into the semiconductor wafer is a sharply varying function of the ratio of the thickness of the resonator to that of the film if trapping is not present, but not if trapping is present. The treatment is very cumbersome to use and is restricted to the case of strip electrodes and diaphragms. In this work a perturbation procedure for the calculation of the Q due to radiation into the wafer is presented, which is considerably less cumbersome to use than the earlier treatment and is not restricted to the case of strip electrodes and diaphragms. The resonant mode of interest is determined from an equation for transversely varying thickness modes in composite resonators and simple approximate but accurate conditions at the edges of the diaphragm, from which the radiation into the wafer is obtained using a variational approximation procedure. The resonant mode and resulting radiation field are employed in a perturbation integral to calculate the Q. For the case of strip electrodes and diaphragms the calculated results are shown to be in good agreement with the earlier more cumbersome calculations. In addition the perturbation calculations are performed for the case of rectangular electrodes and diaphragms.

Thermal analysis of optical elements and arrays on thick substrates with convection cooling
View Description Hide DescriptionBy solving the heat equation for a Gaussian heating source on the surface of a thick substrate with forced cooling, the relative desirability of conductive and convective cooling strategies for removing heat from single elements and arrays has been investigated. Small sources or arrays are most efficiently cooled by diffusion into a thick heat sink and here temperature is not critically dependent on cooling strength. As the arrays get larger, however, the relative efficiency of conductive heat removal is less and it is desirable to maximize convective heat transfer instead. So large arrays require very thin substrates and strong convective cooling.

Photothermal waves in anisotropic media
View Description Hide DescriptionThe coupled 3‐D thermoacousticequations, describing wave propagation in anisotropicsolids, are derived. While solutions to these equations predict the behavior of both thermal and acoustic waves, a specific formulation is presented for the analysis of thermal wave generation, and propagation, in the decoupled mode. The approach is based on a transfer function formalism, and can easily be adapted to accommodate external sources with arbitrary spatial distribution. Simulations pertaining to the case of thermal wave propagation in a semi‐infinite sample, as well as a slab of material are presented. Results predict anisotropy‐induced asymmetric field formation and thermal beam‐steering effects.

Coupled electron and excited‐state kinetics in a nitrogen afterglow
View Description Hide DescriptionThe coupling between the metastable state N_{2}(A ^{3}Σ^{+} _{ u }) and the electron energy distribution function (EDF) as well as the vibrational distribution N_{2}(X,v) of the ground state in a nitrogen afterglow has been studied by simultaneously solving the Boltzmann equation for the EDF and the rate equations describing the kinetics of N_{2}(A) and N_{2}(X,v). The results show that in the decaying plasma, the presence of N_{2}(A) strongly affects both the EDF and N_{2}(X,v). In particular, superelastic electronic collisions produce a broad maximum in the EDF which follows the temporal evolution of the N_{2}(A) concentration, while bimolecular reactions involving N_{2}(A) create a N_{2}(X,v) distribution characterized by a plateau which decays with time.

Ionization‐drift turbulence in rf magnetron plasmas
View Description Hide DescriptionElectron density fluctuations in a rf hollow magnetron system are investigated. The weakly ionized and axially magnetized (B∼80 G) discharge supports nonlinear ionization‐drift waves in the presence of the nonuniform rf‐induced dc electric field and density gradient along the normal of the hollow electrodesurface. The fluctuations have good coherence along B and are turbulent in the transverse plane. The turbulence is inhomogeneous along the density gradient. The power spectrum shows two major bands. The amplitude of the high‐frequency oscillation is modulated by the low‐frequency oscillation. In the transverse plane, the phase velocity is about the same as the diamagnetic drift velocity. It has one component along the diamagnetic drift direction and the other toward the electrodesurface. The turbulence also induces anomalous cross‐field electron current which deteriorates the electron confinement in the low‐pressure regime. Generally, the system combines the nature of the reaction diffusion and the drift systems. The ionization process, the nonlinear transverse drifts under the nonlinear background, and the nonlinear wave‐wave interactions drive the system to turbulence. The parametric dependence of the turbulence is studied and discussed in the paper.

Determination of two‐dimensional temperature and additive density distributions in a high‐intensity‐discharge lamp arc
View Description Hide DescriptionDemixing of additive species can lead to significant axial and radial nonuniformities in the electrical and radiative characteristics of a vertical high‐pressure arc. An optical emission method has been developed to determine both the radial and axial distributions of temperature and additive density as well as the pressure in a rotationally symmetric, enclosed arc in local thermodynamic equilibrium. The method can be used if one knows the vaporized quantity of the major component of the gas mixture in the arc tube. The procedure has been demonstrated by applying it to an ac 175‐W Na‐Sc‐Hg lamp arc, with mercury the major component and sodium the additive of interest. An optically thin Hg line and an optically thin portion of the broadened NaD line were used. The resulting distributions clearly show the effects of segregation; the upper portion of the arc and the axial regions are significantly depleted of sodium.

Angular distribution of x‐ray radiation from optically thick z‐pinch plasmas
View Description Hide DescriptionA z‐pinch plasma is an x‐ray radiationsource of small size concerning the z‐direction radiation. However, the plasma is optically thick with respect to the resonance lines, which contain a large fraction of the total radiation. The influence of the opacity on efficiency and spatial radiation distribution has been examined. A computer simulation solving the radiation transport equation demonstrates that the efficiency of the x‐ray output decreases due to absorption, but only slightly because photon trapping is mostly followed by reemission. Using this result another computer code could determine the following features concerning the angular radiation distribution: (1) The average r‐direction intensity is larger than the z‐direction intensity. (2) The ratio of z‐direction intensity to r‐direction intensity varies from shot to shot. (3) The standard deviation of z‐direction intensity is larger than that of the r‐direction intensity. Several plasma focus experiments were carried out, the results of which provide good qualitative proof of the above predictions.

Electron spin resonance investigation of ion beam modified amorphous hydrogenated (diamondlike) carbon
View Description Hide DescriptionElectron spin resonance(ESR) measurements on diamondlike carbon films show that the as‐grown material possesses a very high (2.5×10^{2} ^{0} cm^{−} ^{3}) concentration of dangling bonds. Upon irradiation with 50‐keV C^{+}carbon ions, the number of ESR active centers increases by a factor of 3.5 and the linewidth narrows, but no shift in the g value is observed, and the resonant peak remains Lorentzian. These effects are concomitant with the previously observed dramatic decrease in the electrical resistivity (4–5 orders of magnitude). The ESR results verify that no graphitelike islands have formed as a result of the irradiation. The likely conduction mechanism is via hopping in band tail states, the number of which increases as a result of the ion impact.

The mechanism of polymer alignment of liquid‐crystal materials
View Description Hide DescriptionSmetic and nematic liquid‐crystal materials can be homogeneously aligned by buffed thin films of appropriate polymers. We propose that the buffing process orients the polymer’s molecular chains in a manner similar to cold drawing of bulk polymer samples. Experimental verification of this theory is obtained by measuring buffing‐induced birefringence in thin films of various polymers coated on glass. Further experiments establish that the oriented state of the polymer chains, and not scratching or grooving of the surface, is necessary to produce alignment. Alignment is found to occur when the polymer is both oriented and crystalline. A picture of alignment is presented in which the formation of a liquid‐crystal phase on the crystalline,oriented polymer surface is analogous to the epitaxialgrowth of conventional solid crystals.

Radiation damage of gallium arsenide induced by reactive ion etching
View Description Hide DescriptionRadiation damage induced in the surface of gallium arsenide(GaAs) by reactive ion etching (RIE)is studied using physical and electrical analyses. The number of displaced Ga and As atoms observed by Rutherford backscattering spectrometry aligned spectra is 7.8×10^{1} ^{5} cm^{−} ^{2} and 1.3×10^{1} ^{6} cm^{−} ^{2} at rf power of 300 W (0.47 W/cm^{2}) and 500 W (0.78 W/cm^{2}), respectively. The barrier height of the Schottkyelectrode formed on the undamaged layer is 0.745 V. The height decreases with increasing rf power and reaches 0.47 V at 0.78 W/cm^{2}. Carrier concentration at the surface of the GaAs channel layer decreases with increasing rf power. The damage can be reduced by annealing at 400 °C for 30 min. For instance, the barrier height at 300 W increases from 0.47 to 0.70 V, and the carrier concentration increases from 7.0×10^{1} ^{6} to 1.3×10^{1} ^{7} cm^{−} ^{3}. However, the carrier concentration cannot recover to the same level as that of an undamaged layer with annealing at 400 °C. The temperature conventionally employed in GaAselectrodeannealing process.

Enhanced elimination of implantation damage upon exceeding the solid solubility
View Description Hide DescriptionImplantation of silicon wafers with Ga and P, under specific conditions, results in e n h a n c e d category‐II (end of range) dislocation loop elimination after short thermal cycling. Comparison of these results with transmission electron microscopy studies of Si‐, Ge‐, As‐, Al‐, and Sb‐implanted samples indicate that the enhanced elimination process occurs only when the peak of the impurity concentration exceeds the solidsolubility of the impurity in silicon at the annealing temperature a n d the resulting precipitates are dissolving. The activation energy for enhanced elimination of these extrinsic catetory‐II dislocation loops is shown to be 5±0.5 eV. It is proposed that vacancy emission by the dissolving precipitates is responsible for the enhanced elimination.

Formation of buried nitride silicon‐on‐insulator structures studied by Auger electron spectroscopy and transmission electron microscopy
View Description Hide DescriptionThe formation of buried nitride silicon‐on‐insulator structures is studied by means of Auger electron spectroscopy and transmission electron microscopy. 160‐keV N^{+} ions are implanted to doses of 7.0 and 9.5×10^{1} ^{7} atoms cm^{−} ^{2}, respectively, at a substrate temperature of 550 °C. Annealing between 1150 and 1200 °C removes most of the damage in the top silicon layer in ≤ 1 h. In ≤ 13 h the buried layer turns into crystalline Si_{3}N_{4}. This crystallization process starts at a depth where the N concentration is well below the value for Si_{3}N_{4}.

A probabilistic model of brittle crack formation
View Description Hide DescriptionProbability of a brittle crack formation in an elastic solid with fluctuating strength is considered. A set Ω of all possible crack trajectories reflecting the fluctuation of the strength field is introduced. The probability P(X) that crack penetration depth exceeds X is expressed as a functional integral over Ω of a conditional probability of the same event taking place along a particular path. Various techniques are considered to evaluate the integral. Under rather nonrestrictive assumptions we reduce the integral to solving a diffusion‐type equation. A new characteristic of fracture process, ‘‘crack diffusion coefficient,’’ is introduced. An illustrative example is then considered where the integration is reduced to solving an ordinary differential equation. The effect of the crackdiffusion coefficient and of the magnitude of strength fluctuations (ratio of minimal and mean values of the strength field) on probability density of crack penetration depth is presented. Practical implications of the proposed model are discussed.

Deformation behavior of undoped and In‐doped GaAs in the temperature range 700–1100 °C
View Description Hide DescriptionCompressive deformation of undoped and In‐doped GaAs single crystals has been carried out in [001] and [123] orientations in the temperature range 700–1100 °C. Indium additions, at levels of 1–2×10^{2} ^{0} atoms cm^{−} ^{3}, result in critical resolved shear stress (CRSS) values that are about twice as large as the undoped crystals in the temperature range of 700–1100 °C. The CRSS was weakly dependent on temperature in the temperature range investigated as expected for a model of athermal solid solution hardening. The CRSS value of 3.3 MPa for the In‐doped crystal is sufficient to eliminate profuse dislocation formation in a 75‐mm‐diam crystal on the basis of current theories for the magnitude of the thermal stress experienced during growth. The results also suggest that the process of dislocation climb is slowed appreciably by In doping.

Bulk properties of composite media. II. Evaluation of bounds on the shear modulus of suspensions of impenetrable spheres
View Description Hide DescriptionWe evaluate third‐order bounds due to Milton and Phan‐Thien on the effective shear modulusG _{ e } of a random dispersion of identical impenetrable spheres in a matrix up to sphere‐volume fractions near the random close‐packing value. The third‐order bounds, which incorporate two parameters, ζ_{2} and η_{2}, that depend upon the three‐point probability function of the composite medium, are shown to significantly improve upon the second‐order Hashin–Shtrikman (or, more general, Walpole) bounds which do not utilize this information, for a wide range of volume fraction and phase property values. The physical significance of the microstructural parameter η_{2} for general microstructures is briefly discussed. The third‐order bounds on G _{ e } are found to be sharp enough to yield good estimates of the effective shear modulus for a wide range of sphere‐volume fractions, even when the individual shear moduli differ by as much as two orders of magnitude. Moreover, when the spheres are highly rigid relative to the matrix, the third‐order lower bound on the effective property provides a useful estimate of it. The third‐order bounds are compared with experimental data for the shear modulus of composites composed of glass spheres in an epoxy matrix and the shear viscosity of suspensions of bituminous particles in water. In general, the third‐order lower bound (rather than the upper bound) on G _{ e } tends to provide a good estimate of the data.